The Foreign Office main building + HM Treasury building. Two behemoths standing on the edge of Whitehall, housing two of the three Great Offices of State (the other being the home office), and more importantly for me, covered in beautiful architecture.
These buildings are already nearing their one year anniversary but they hold a small place in my heart as the first large scale, solo builds I took on. And despite procrastinating for about a month (and nagging from nearby builders), convinced myself to complete them.